In general, Mn, Cr, Mo, Ni, and the like are added in combination to a steel used for mechanical and structural components. A steel for carburizing which has the chemical composition and is produced by casting, forging, rolling, and the like is subjected to shaping such as forging and machining which is cutting and the like and subjected to heat treatments such as carburizing and the like, and then the steel for carburizing becomes a carburized steel component with a carburized layer which is a hardened layer in a surface layer and a steel portion which is a base metal that is not influenced by the carburizing treatment.
In producing cost of the carburized steel component, cost for the cutting is particularly high. The cutting is disadvantageous to a yield, because tools for the cutting are not only expensive, but also the cutting forms a large amount of chips. Thus, replacing the cutting with the forging is attempted. The forging method is divided roughly into a hot forging, a warm forging, and a cold forging. The warm forging has a feature in which scale formation is not much and dimensional accuracy is improved as compared with the hot forging. The cold forging has a feature in which the scale formation is little and the dimensional accuracy is close to the cutting. Thus, it is tried that the cold forging is performed as a finishing after the hot forging is performed as a rough shaping, that the cutting is slightly performed as the finishing after the warm forging is performed, or that the cold forging is only performed for the shaping. However, since mold life decreases with increase in contact pressure to the mold in a case that deformation resistance of the steel for carburizing is large when replacing the cutting with the warm forging or the cold forging, advantage of the cost against the cutting becomes small. Or problems such that cracks are initiated and propagated at an area where large deformation is applied and the like occur when forming into complex shape. For the reason, various techniques have been investigated in order to soften the steel for carburizing and to improve critical working ratio.
For example, Patent Documents 1 and 2 suggest the steel for carburizing which is softened by decreasing Si and Mn content in order to improve cold forgeability. The steels for carburizing have sufficient hardness of steel portion and effective case depth (depth where Vickers hardness is HV550 or more) after the carburizing and have properties satisfied as the carburized steel component. However, it is insufficient to decrease drastically the deformation resistance at the forging. In contrast, Patent Document 3 suggests the steel for carburizing in which the deformation resistance at the hot forging, the warm forging, and the cold forging is drastically decreased by decreasing considerably C content to 0.001% to 0.07% or less as compared with the conventional steel for carburizing and in which effective hardened layer after the carburizing that is reduced due to the decrease in C content is improved by controlling the amount of additive elements except C. However, the hardness of the steel for carburizing decreases by excessively low C content as the steel, and the hardness of steel portion of the carburized steel component which is not influenced by the carburizing is insufficient. Therefore, a problem such that versatility has restriction occurs. Patent Document 4 suggests the steel for carburizing which is excellent in ductility and is able to be utilized for the cold forging with large working ratio by improving metallographic structure of the surface layer of the steel for carburizing whose shape is a bar and wire rod by spheroidizing annealing. The critical working ratio of the steel for carburizing is improved, and the cracks which are initiated and propagated at the cold forging can be prevented. Moreover, the steel for carburizing has satisfiable properties as the carburized steel component in regard to the hardness of steel portion and the effective case depth after the carburizing. However, the steel for carburizing is ineffective in decreasing in the deformation resistance at the forging, and an improvement such as a decrease in forging load, a prolongation of the mold life, and the like should be performed.
As mentioned above, it is fact that the technique satisfying all properties such as the drastic decrease in the deformation resistance at the forging, the improvement of the critical working ratio, the securement of the properties as the carburized steel component, and especially the securement of the effective case depth and the hardness of steel portion is not found.